1. Technical Field
Improved techniques and equipment for the calcination of calcium sulfate dihydrate (sometimes known as gypsum or land plaster in the naturally occurring form, as syngyp in the synthetically derived form, or by the chemical formula CaSO4.2H2O) to primarily the alpha type of calcium sulfate hemihydrate (CaSO4.½H2O[alpha type]) are disclosed.
2. Description of the Related Art
Gypsum and calcium sulfate-based compositions and compounds are used in a wide variety of industries, including the construction industry. Calcium sulfate dihydrate is a naturally occurring mineral that can be mined. Additional sources of calcium sulfate dihydrate include synthetic gypsum generated from flue gas desulfurization of coal burning power plants (FGD gypsum) and various sources of recycled gypsum such as recycled wall board and gypsum recycled from casts or molds.
To render gypsum useful as a construction material, it can be calcined or heated to partially dehydrate it to the alpha and beta forms of calcium sulfate hemihydrate. The dehydration or calcination of raw gypsum to calcium sulfate hemihydrate can be represented by the following formula:CaSO4.2H2O+heat→CaSO4.½H2O+3/2H2O
Calcination is the process where calcium sulfate dihydrate is converted to hemi-hydrate, soluble anhydrite, and/or insoluble anhydrite. A number of different techniques may be used to calcine gypsum. Calcination may be performed, for example, by flash drying at high temperatures, cooking in large kettles, heating in furnaces or rotary kilns, using steam, or cooking in aqueous suspensions. These different techniques can result in calcium sulfate products with a wide range of composition and properties, but generally two types of hemihydrates are formed: alpha-hemihydrate type and beta-hemihydrate type.
In a reversal of the calcination process described above, the hemihydrates are dissolved in water until they are saturated and the soluble hemihydrates are exothermically converted back to the less soluble dihydrate, which precipitates out of the solution thereby further driving the following reaction:CaSO4.½H2O+3/2H2O→CaSO4.2H2O+heat
As the amount of dihydrate produced increases, the gypsum sets. The setting of the gypsum can be followed by measuring the heat evolved, represented by a gradual increase in slurry temperature.
Methods of improving the thermal efficiency of calcination of the hemihydrate have been accomplished, but typically rely on the use of an aridizing agent, such as the deliquescent salt CaCl2, which acts to lower the calcining temperature. However, use of CaCl2 as an additive is better suited to batch processes than continuous processes. Further, the addition of a salt can be deleterious to wallboard qualities such as plastic flow and the bond of paper to the gypsum core.
Alpha and beta hemihydrates can be distinguished from one another by the amount of water that is necessary to make a pourable slurry with the finely ground powdered hemihydrate. Alpha calcium sulfate hemihydrate, also referred to as alpha-hemihydrate requires less than about 50 mL per 100 g of plaster, while beta calcium sulfate hemihydrate, also referred to as beta-hemihydrate, requires substantially more water, normally more than 70 mL per 100 g of plaster. This amount of water is known as the “water demand.” A high water demand, sometimes reflected as a water/stucco ratio (W/S), is less efficient from a wallboard production standpoint, since more energy is needed to remove the excess water in the board drying process. A large part of the energy is in increased fuel costs. Conventional beta-hemihydrate is more porous, and has a W/S ratio in the range of 0.7-0.8, while alpha-hemihydrate has a W/S ratio in the range of 0.32-0.45.
The alpha and beta forms of the hemihydrates also differ in crystal shape and density. Also, while alpha-hemihydrate, in its B-base form is typically made in a batch system, beta-hemihydrate is made in a continuous system. Despite the latter advantage, because of the lower water demand, alpha-hemihydrate is preferred over the beta-hemihydrate. However, current calcining techniques are relatively ineffective at producing the alpha-hemihydrate content of the calcium sulfate product. As a result, conventional gypsum wallboard manufacturers use beta-hemihydrate, produced by kettle or flash calcination under ambient pressure.
Employing alpha-hemihydrate in wallboard manufacturing would represent a significant savings in energy and production costs. Because the W/S ratio is much lower with alpha-hemihydrate, the fuel (typically natural gas) used to dry the board is reduced, and the wallboard production line speed can be increased, thus increasing production capacity. The combustion related polluting gas emissions are also reduced.